the.com/ace in the hole

The advantage you mention casually right after you were sure you'd lose.

means A hidden advantage or resource kept secret until the moment you need it most.

from From stud poker, where one card is dealt face-down ('in the hole') and hidden from other players until showdown. If that hidden card happened to be an ace, it could secretly win the whole hand — the best possible card, held in reserve, unseen until it mattered. The phrase moved from card tables into everyday speech by the early 20th century, keeping its sense of a secret trump card saved for the right moment.

origin
from stud poker, one card dealt face down
the trick
opponents can't price in what they can't see
misuse warning
not every backup plan deserves the name
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